My biggest has been 13900x16000. Not quite as large as yours but mine was 107 layers. Took around 90 hours with one 2.53GHz P4, one 2.2GHz Athlon and one 3.46GHz dual-core Intel 955 Extreme working together in a network render. I only render that big if it's something I'm going to have printed. No need for it to be that big otherwise.

 

Rick